Both flexible and rigid polyurethane foams have found wide use, but their flammability, with resulting toxic emissions has caused considerable difficulties. This is especially so for flexible foams which, on burning, depolymerise and melt to form a liquid which flows, burns, volatalises and causes flashover, ie the formation of a sudden sheet of flame.
In an attempt to avoid these difficulties, many flame retardants have been incorporated in the polyurethane foam, and a variety of mechanisms operate in which the retardants may act as any one or more of thermal barriers, depolymerisation inhibitors, flow inhibitors and moisture sinks.
Fire retardants which have been tried to date include inorganic additives, such as cement, graphite, talc, calcium oxide, hydrated alumina, silicates, silica and phosphorus compounds, and organic additives such as halogen compounds and melamine. The use of a mixture of inorganic and organic fire retardants is also known; see for example JP-A-53-26898 which describes the use of an alkali silicate powder, together with an organic halogen compound as flame retardant.
Some additives adversely affect the polymerization reaction and/or foaming process, especially where the additive carries water. In particular, the water tends to react with the isocyanate in the reaction mixture. There are also some additives which seriously impair the mechanical and other properties of the foam. For example, although hydrated alumina acts as an efficient water sink, it produces an adverse feel and prevents the formation of a low density foam, while graphite, in addition to being expensive, gives a black colouration. Other additives which we thought might prove useful, for example, cement, although inhibiting the onset of ignition, actually promote burning because, although they form a rigid char, this acts as a wick for the flame. Thus weight and heat loss are actually increased.
Likewise, melamine (a flame retardant which is currently popular) delays the onset of ignition. However, once ignition occurs, weight loss is actually no less than for a standard foam containing no fire retardant additive. Toxic emissions, including cyanides are also generated.
As an example of the above known flame retardants, it has long been known to use small particles, ie up to 60 mesh, of alkali metal silicate to inhibit the onset of ignition; see for example JP-A-52-17598.
U.S. Pat. No. -A-4871477 discloses the use of a water soluble alkali metal silicate powder in which water is retained. At fire temperatures, the silicate becomes fusible and water in the particles volatilises and expands the particles to produce rapidly an inorganic silicate foam which ultimately releases water, contributes to the formation of the barrier layer and has significant fire resistant properties. These particles have a maximum diameter of 60 microns. In the formulations specifically disclosed, the powder is employed in combination with "microexplosive" particles containing a compound capable of generating a gas, and in addition a phosphate flame retardant.